(1) Field of the Invention
This invention relates generally to the field of supply voltage generators (linear regulators or DC-DC converters) and more specifically to those equipped with an input current limit control mechanism.
(2) Description of the Prior Art
Every modern integrated power management system has to be able to accommodate for a broad range of voltage sources (USB, 5V wall adapter, Firewire, automotive battery). Each of them comes in a variety of output specs, in particular regarding nominal output voltage and maximum current capability. The power management unit (PMU) has to guarantee that in every circumstance the load seen by the power source is within the specified ranges. This is generally done imposing an input current limitation on the PMU according to the kind of power source connected to it.
In FIG. 1 prior art a generic schematic of such a power management system is shown. The power source is represented by an ideal voltage source Vpwr with its intrinsic output resistance in series. The power source is then connected to the PMU through a connection cable whose length and resistance are not generally known a priori. The PMU itself is represented by a black-box system which is in charge of transferring the power from the source to a generic battery operated system, here represented by an ICHG load (current required to charge the battery) and an ISYS load (current required for the other system components).
In this scheme the voltage at the PMU input Vin is constantly monitored by a comparator. If the input voltage Vin falls below a certain threshold Vth, which represent the minimum limit specified for the particular power source connected to the PMU input, the power transfer between the power source and the system/battery is interrupted by disabling of the PMU.
This prior art scheme shows an undesired behavior in presence of high resistive interconnect (high Rcable due to long interconnection cable) or low budgetary power source (high source output resistance Rout). In this case even an input current below a specified maximum (Iin<Imax) leads the PMU input voltage to fall below the minimum voltage threshold Vth and by consequence to a disabling of the PMU. This event in turn reduces abruptly the input current to zero, thus allowing Vin to rise above the minimum voltage threshold again. The PMU will be re-enabled, the input current will increase again and the described process will repeat itself. This behavior is obviously unacceptable: even a load current generating an input current below the specified limit can lead to an interruption of the power flow and causes startup or operation failures. Furthermore the continuous PMU toggling between ON and OFF state will generate high frequency voltage/current transients that could interfere or damage other system components.
A typical situation reproducing a phenomenon like the one described above is the case in which the PMU is connected via a 1-Ohm cable to a high-power USB port. Assuming the USB voltage to be Vpwr=5.0V and the minimum voltage threshold to be Vth=4.4V (lower limit of the USB range), a charge current Ichg=1.0 A would reduce the voltage at the PMU input to Vin=4.0V thus leading to the PMU turnoff (the source output resistance has been neglected in this example).
There are patents or patent publications dealing with the operation of buck converters.
U.S. patent (U.S. Pat. No. 7,262,585 to May) discloses a power supply system having a transistor, a linear regulator, a DC-DC converter, and a control circuit. The transistor has an input, a substrate, a first node, and a second node. The first node is operably coupled to a non-battery power source. A linear regulator is operably coupled to the second node to produce a regulated output voltage based on the non-battery power source, when enabled. A DC-DC converter is operably coupled to produce the regulated output voltage based on a battery power source, when enabled. A control circuit is operably coupled to the input node and the substrate of the transistor wherein when the DC-DC converter is enabled, the control circuit controls a reverse leakage current of the transistor, and when the linear regulator is enabled in a zero load-state, the control circuit controls a forward leakage current of the transistor, and when the linear regulator is enabled in a non-zero load-state, the control circuit provides a current limit for the linear regulator.
U.S. patent (U.S. Pat. No. 7,254,044 to Perry et al.) proposes various embodiments of a power supply all including at least one DC/DC converter. The converter includes a primary switch controlled by a pulse width modulated control signal such that the primary switch is on for a D time period of each switching cycle of the converter and is off for a 1-D time period of each switching cycle. Also, the power supply includes a current sensing element connected in series with the primary switch. In addition, the power supply includes a current limit circuit connected to the current sensing element. The current limit circuit includes a functional circuit having a first input responsive to a first signal whose voltage is proportional to the output current of the converter during the D time period of the switching cycle of the converter. A second input of the functional circuit is responsive to a second signal whose voltage is proportional to the output current of the converter during the 1−D time period of the switching cycle of the converter. In that way, the voltage of the output signal of the functional circuit is proportional to the output inductor current of the converter over both the energy storage phase (the D interval) and the energy deliver phase (the 1−D) interval of the converter.
U.S. patent (U.S. Pat. No. 4,263,644 to Zellmer) discloses a switched DC-to-DC converter in a power supply being powered by input line current from an external power source and driven by voltage pulses from a variable duty cycle pulse width modulator for converting a DC input voltage to a DC supply voltage of a different value that is applied to a load impedance. A comparator monitors the supply voltage for producing an error voltage that biases the modulator for adjusting the width of the voltage pulses, and thus the duty cycle of the converter, for maintaining the supply voltage relatively constant. An RC circuit integrates the voltage pulses for producing an indication of the average value thereof, which is directly related to the value of line current drawn by the converter. When the average value of voltage pulses exceeds a reference voltage, the value of bias voltage is limited for establishing the maximum width of voltage pulses and duty cycle of the converter, and thereby limit the maximum line current drawn by the power supply.
U.S. patent (U.S. Pat. No. 7,414,377 to Mayhew et al.) describes a motor controller system comprising solid-state switches for connection between an AC line and motor terminals for controlling application of AC power to the motor. A sensor senses AC line voltage. A control circuit controls operation of the solid-state switches. The control circuit ramps switch current during a start mode and selectively holds switch current during the start mode if sensed voltage drops below a threshold amount.
Furthermore Texas Instruments has published an application note “Fully Integrated Switch-Mode One-Cell Li-Ion Charger with Full USB compliance and USB-OTG support” describing a charge management device for single cell batteries, wherein charge parameters can be programmed through an I2C interface. The bQ24150/1 charge management device integrates a synchronous PWM controller, power MOSFETs, input current sensing, high accuracy current and voltage regulation, and charge termination, into a small WCSP package.